A bipolar-type ion exchange membrane electrolyzer is currently known as an ion exchange membrane electrolyzer, in which bipolar elements each integrating with an anode chamber harboring an anode on one surface and a cathode chamber harboring a cathode on the other surface are arranged side by side with an ion exchange membrane interposed between the bipolar elements. In this type of electrolyzers, an electrolyzer is composed of tens of bipolar elements arranged in series with an ion exchange membrane interposed between the respective bipolar elements and plural electrolyzers of this type are electrically interconnected and the resulted electrolyzer system is operated.
A leakage current flows through the laying pipe of each bipolar element, while electrolytes are circulating through pipes for supplying anode solution and for supplying cathode solution and pipes for discharging anode solution and for discharging cathode solution in operation of an electrolyzer. In the electrolysis of brine, for example, an electromotive force is induced across an intervening ion exchange membrane in a unit cell after stopping operation of an electrolyzer by virtue of an active material in an anode solution inside an anode chamber, chlorine (Cl2), and an active material in a cathode solution inside a cathode chamber, hydrogen (H2), and a leakage current flows through laying pipes while stopping the operation as well as while running the operation. Therefore, an electrical current (reverse current) flows in the electrolyzer in a direction opposite to that of current flow during positive electrolysis. A variety of measures are adopted with the aim to reduce reverse current flow, because these reverse currents result in performance degradation in an electrolyzer, such as degradation of a cathode and the like.
A technology has been proposed as a general procedure to prevent a reverse current, for example, in Patent Document 1, in which brine is injected in an anode chamber to eliminate an active material inside the anode chamber, chlorine (Cl2), after stopping operation of a brine electrolyzer. According to this procedure, the concentration of the active material in the electrolyte is decreased and the electromotive force to generate a reverse current is thereby reduced. Moreover, the temperature of the electrolyte is involved as a cause of enhancement in electromotive force to generate a reverse current. An electromotive force to generate a reverse current is reduced as the temperature of the electrolyte is decreased, in particular, to around room temperature, thus the temperature of electrolyzer is preferably low, and the injection of brine to the anode chamber also serves to reduce the temperature of the electrolyte. Furthermore, Patent Document 2 proposes a technology as a method of discharging a residual voltage in a water electrolysis cell, in which residual oxygen (O2) in an anode chamber and residual hydrogen (H2) in a cathode chamber are eliminated by continuing supply of an electrolyte, i.e., pure water to both the anode and cathode chambers.
Patent Document 3 proposes a technology as another technique with the aim to reduce reverse current flow through an electrolyzer, in which an application of a direct current voltage between an anode and a cathode after stopping operation of an electrolyzer causes a minute current (protective current) to flow from the anode to the cathode and thereby prevents reverse current. Furthermore, electrodes less susceptible to reverse current are developed. For example, a cathode for aqueous solution electrolysis is proposed in Patent Document 4, in which a mixed layer of metal nickel, nickel oxide and carbon atom is formed on the surface of an electrically conductive substrate with nickel surface and then a catalytic layer of electrode containing a metal(s) of platinum group or a oxide(s) of platinum group metal(s) is configured on this mixed layer. Moreover, Patent Document 5 proposes a method of reducing a reverse current in which the length and bending positions of a hose, which connects an outlet for cathode solution to a cathode solution recovery pipe, are optimized.